The federal government will extend protections to all imperiled Canada lynx in the lower 48 U.S. states, but wildlife advocates said on Thursday it was ignoring important parts of the rare cats' range and they vowed to challenge the move in court.

Thickly furred lynx roam through high country from Maine to Washington and south through the Rocky Mountains. They are classified as threatened in the lower 48 states under the Endangered Species Act.

Amid calls from conservation groups for greater protections, the U.S. Fish and Wildlife Service said in a rule to be published on Friday that it was extending the Act's protections to all lynx "where found" in the contiguous United States.

The law broadly bans killing or injuring imperiled animals without a special permit.

"Our final rule provides the lynx, one of only seven wild cat species in the nation, with what it needs to persist and thrive for future generations of Americans," said Noreen Walsh, the Service's Regional Director for the Mountain Prairie Region.

Separately, the service is designating some 38,954 square miles (100,891 square km) in Idaho, Maine, Minnesota, Montana, Washington and Wyoming, which it said "constitute our current best assessment of the areas that meet the definition of critical habitat" for lynx.

WildEarth Guardians, one of the groups which has sued the federal government over lynx protections in the past, applauded the fact the new rule extended safeguards to all lynx, including a population in New Mexico that had been overlooked.

But it said it was disappointed over the separate issue of critical habitat designation, saying the federal authorities had reduced the overall area by 2,593 square miles (6,716 square km) from what was proposed by the Service last year.

A designation of critical habitat gives greater protections to environments which threatened species rely on to survive, and imposes restrictions on activities such as mining, logging and snowmobiling in the high country where lynx are found.

Drew Kerr, a carnivore advocate for WildEarth Guardians, said the new designation ignored important parts of the cats' range extending from southern Wyoming, through Colorado, to northern New Mexico. Sections of Washington state, Oregon, Idaho and Montana were also excluded, he added.

His group and the Western Environmental Law Center (WELC) plan to challenge the "inadequate" designation in federal court.

"By ignoring huge swaths of currently occupied lynx habitat, the Service is undermining lynx recovery efforts yet again," Kerr told Reuters.

A Fish and Wildlife Service spokesman said it was the service's policy not to comment on possible lawsuits.

Conservation groups sued Idaho wildlife managers and the governor in June, saying the state violated the Endangered Species Act by allowing traps and snares set by hunters in habitat favored by lynx.

A similar lawsuit was filed against Montana last year, contending that at least nine of the cats had been illegally caught because of trapping and snaring aimed at animals such as wolves and bobcats.